Soil Nutrient Retention and Yield Effect of Nitrogen, Phosphorus Synergists on Wheat/Maize Rotation in Brown Soil

: The aim was to improve the fertilizer utilization efﬁciency and alleviate environmental pollution risk under a wheat-maize rotation system. Here, the combinations of different nitrogen stabilizers and phosphorus activators were used to reduce nitrogen loss and phosphorus ﬁxation in the ﬁeld experiment. Compared to the control, the combination of 1.5%HQ + 0.5%DMPP + biochar showed the most signiﬁcant effect on the retention of alkali-hydrolysable nitrogen (Nah), the highest with an increase of 22.6% at the 0~20 cm layer soil; and the combination of 1.5%HQ + 3.5%DCD + CMFs (compound microbial fertilizers) showed the most signiﬁcant effect on the maintenance of available phosphorus (Pa), with the highest increase of 41.3%. N, P synergists combined with a basal fertilizer could effectively slow down the transformation from NH + 4 to NO − 3 , and keep NH + 4 at an increase of 7.38%~19.6%. Moreover, the N, P synergists could efﬁciently lock the available nutrients around the roots, preventing the migration of NO − 3 , NH + 4 , Nah, and Pa to the deeper layers. Especially for NO − 3 , the total accumulation at 0~60 cm decreased by 32.1%, and the activation of Pa was mainly concentrated at 0~40 cm. Under the same nutrient inputs, the combination of 0.3%NBPT + 0.5%DMPP + CMFs obtained the highest wheat yield. The combination of 1.5%HQ + 0.5%DMPP+ biochar gained the highest maize yield. Overall, the application of N, P synergists could increase the effective duration of Nah, Pa, and NH + 4 in the surface soil, and reduce the accumulation of NO − 3 in the 0~60 cm soil layer. The capacity of holding and keeping nutrients from leaching rose obviously; simultaneously, the assimilative capacity of crops for nitrogen and phosphorus increased distinctly, which could lower the eutrophia risks from nitrogen and phosphorus.


Introduction
Nitrogen (N) and phosphorus (P) are the most crucial elements for crop yield, but available form deficiency widely exists in agricultural practices, thus supplying a chemical fertilizer is one of the most effective measures to increase the grain yield. In order to pursue the sustainable high yield of food crops, massive fertilizers are applied into agricultural soil. However, due to limited education level and the cognition of the farmers or employees, the phenomena of blind fertilization and the over-fertilization or overdose are widespread in practice, which leads to a serious imbalance between the available nutrients provided by the soil and the actual demand of the crops [1,2]. Whether as basal application or topdressing, nitrogen (N) fertilizer is the most commonly used in practice. If the available nitrogen could not be absorbed and transformed in time by crops, it is easy to volatilize as NH 3 , leach as NO − 3 , or denitrificate as N 2 O or N 2 [3][4][5][6][7]. The proportion of nitrogen in basal fertilizer is generally as high as 50-70% in the wheat/maize rotation of the Huang-Huai-Hai Plain, and urea is the main nitrogen fertilizer source. Studies have found that it takes 7-10 days, 4-5 days, and 2 days to convert urea into N 2 O or N 2 at 10 • C, 20 • C, and 30 • C, respectively. From seeding (October) to regrowth (March), because the climate temperate is commonly low in the test region, the wheat seedings grow slowly and N absorption is low accordingly, as obviously the N supply was far more than the N demand for wheat seedlings [8][9][10][11]. In calcareous soil or cinnamon farming areas in semi-arid areas, NH + 4 is also seriously easy to transform into NO − 3 . On one hand, NO − 3 is easy to leach in the hot and rainy season; on the other hand, NO − 3 accumulation is a key material reason for denitrification, which are both important ways resulting in N loss [12][13][14].
Due to the poor mobility and the strong chemical fixation in soil, the Pa that could be absorbed and utilized by roots is relatively low, although the total phosphorus is generally high. Moreover, the phosphorus fertilizer supply often far exceeds the crop demand. The excessive input of phosphorus fertilizer and unreasonable fertilizing methods led to the utilization rate of phosphate fertilizer of only 10~25%. As phosphorus accumulates in the tillage, soil enters the surface (lower) water body with irrigation or rainfall, which hints at a vital source of the eutrophication of water bodies [12,15].
Urease inhibitors can retard urea converting to NH + 4 by inhibiting the urease activity. Nitrification inhibitors can reduce the generation and accumulation of nitrates by inhibiting the transformation of NH + 4 to NO − 3 , which directly reduces the leaching loss of NO − 3 and denitrification. Biochar, with a developed pore structure, large specific surface area and excellent adsorbent capacity, is considered as a good soil regulator, which could not only improve the soil organic carbon, but also have a strong adsorption capacity on soil NH + 4 and NO − 3 , thus affecting nitrogen transformation [16][17][18]. Other studies have indicated that biochar can adsorb nitrifying inhibitors, delay the release of inhibitors in soil, and reduce nitrogen leaching loss, accordingly, playing a certain protective role on nitrifying inhibitors [19,20]. Biochar itself also contains high phosphorus, and when applied to the soil, it can not only regulate the soil pH and adsorb the complex of soil phosphorus and metal, but also directly act as carbon nutrition for soil microorganisms, improving the bioconversion rate of soil phosphorus, etc. [21,22]. Base-applied CMFs mostly contain a large number of living microbial populations via their growth, reproduction, and activation of metabolites to accelerate the decomposition of organic matter such as returned straw in the soil, while transforming soil-solidified mineral nutrients into a form that can be easily absorbed, so that inherent nutrient resources of the soil can be fully utilized [23]. Both biochar and CMFs could play vital roles as phosphorus activators in practice.
Currently, research on nitrogen stabilizers (urease inhibitors and nitrification inhibitors) and the combined application of biochar, CMFs focused on greenhouse gas emissions [24][25][26][27][28], and the research methods adopted has mainly been based on indoor simulation, and the conclusions varied greatly. Based on the above, the combined application of the two synergists is supposed to have a superimposed effect [6,14].
Here, this research will exploit the spatiotemporal changes and transport characteristics of soil N, P under different combinations of N, P synergists in the field wheat/maize rotation, which will hopefully provide scientific basis and technical support for the efficient utilization of nutrient resources and the practical control of non-point source pollution.

Experimental Field Status
The experiment was conducted in Shiqiang

Experimental Materials and Application Process
The compound fertilizer N-P 2 O 5 -K 2 O was 19-18-18, of which urea accounted for 89.5% and NH + 4 accounted for 10.5%. The raw materials of compound fertilizer were urea (N 46%), monoammonium phosphate (N 14%, P 2 O 5 46%), and potassium sulfate (K 2 O 51%), derived from Shandong Agricultural University Fertilizer Science and Technology Co. Ltd. The biochar originated from Liaoning Biochar Engineering Technology Research Center, and the CMFs from Lianye Biotechnology Co. Ltd (Jiang Yin City, China).
Wheat variety Tainong 18, with a seeding quantity of 164.9 kg·ha −1 , a wide and precise sowing row spacing of 26 cm, and the fertilizer rate for wheat was N 242.6 kg·ha −1 , P 2 O 5 75 kg·ha −1 , and K 2 O 60 kg·ha −1 . After the maize was harvested, all of the straw was returned to the field (about 11,000 kg·ha −1 ), mixed with the basal fertilizer and the synergist thoroughly, and then evenly spread. The required potassium was applied as a basal fertilizer in the form of compound fertilizer at once, and the remaining phosphorus (12.5% of total amount) and nitrogen (50% of total nitrogen) were topdressed with monoammonium phosphate and urea, respectively. The wheat was sown in mid-October and harvested in early June.
Maize variety Zhengdan 958 was tested with the cultivation density 59,000 plants·ha −1 . There was no tillage before the maize season, and the synergist and fertilizer were evenly mixed and sown simultaneously with seeds into the furrow at one time. The test fertilizers were urea, monoammonium phosphate, and potassium sulfate with the quantities: N 282.5 kg·ha −1 , P 2 O 5 45 kg·ha −1 , and K 2 O 76.5 kg·ha −1 , respectively. Maize was sown in mid-June and harvested in early October. The fertilizer formula was both recommended by local cultivation technical procedures.
The field experimental plot was 50 m 2 (5 m width, 10 m length) and spaced 2 m apart. The plot was in a randomly complete block design, and each treatment included three field replications. Agricultural cultivation management was the same as local conventional practice.

Sample Collection and Determination
The 0~20 cm deep soil samples were collected at the wheat seedling stage, heading stage, and maturity stage, respectively, and before maize sowing, the jointing stage, the tasseling stage, and maturity stage. Soil samples of the 0~20 cm, 20~40 cm, and 40~60 cm layers were collected after fertilization (56 d), before wheat harvest (224 d), and before maize harvest (344 d) between the wheat rows and maize plants with soil drill. On smallsized experimental plots, the samples according took the 'W' type, after impurity removal, these were mixed evenly, bagged separately, and taken back to the laboratory. Part of the fresh soil was tested for soil NO − 3 , NH + 4 , and the rest was air-dried naturally, ground, and stored through a 2.0 mm sieve for later use.

Determination of Soil Nutrient Content
Nah was determined by the alkali hydrolysis diffusion method; NO − 3 and NH + 4 were extracted with 2 mol·L −1 KCl and determined by flow analyzer; Pa was extracted with Agronomy 2022, 12, 2445 4 of 16 0.5 mol·L −1 NaHCO 3 (pH 8.5) and determined by molybdenum antimony anti colorimetry. The determination methods referred to Bao [29].

Plant Harvest and Sample Treatment
After the wheat was mature, the yield components of wheat were investigated by the "1 m double row method". Ten representative plants were randomly selected from each plot and divided into three parts (grain, stem, and leaf), washed, bagged, dried at 80 • C to constant weight.
When the maize was harvested, 10 representative plants were randomly selected in each plot and divided into stalk and grains, then washed, bagged, dried at 80 • C to constant weight.
All of the collected samples were ground and mixed evenly to obtain analytical samples and determine the plant nitrogen and phosphorus.

Data Processing
The data in the dynamic figure and column chart about nitrogen in soil were from 2020 to 2021. The crop yield and nutrient accumulation were the mean values of two rotations from 2019 to 2021. Office 2017 and Origin (OriginLab, Mass, USA) were used for the data processing and plotting. The least significant difference (LSD) was used for multiple comparisons between the different treatment means.

Soil NH +
4 under Different Combinations of N, P Synergists in Wheat/Maize Rotation From the variation trend during the wheat season, the NH + 4 content in the 0~20 cm soil layer was the highest at the seedling stage, and decreased to about 50% of the seedling stage at the heading and maturity stage in the next year ( Figure 1A), which was consistent with the trial results in 2019-2021. At the wheat seedling stage, the NH + 4 content treated with T3 (HQ + DMPP + biochar) and T6 (NBPT + DMPP + CMFs) were the highest two, increased by 7.65% and 9.21% compared with CK, respectively. At the wheat heading stage, the NH + 4 levels in all treatments dropped to half of the seedling level, but the soil NH + 4 level under T1, T3, and T6 increased significantly compared with CK, with increases of 35.5%, 45.8%, and 50.1% ( Figure 1A). At the maturity stage of wheat, the content of soil NH + 4 increased slightly compared with the heading stage, while the largest one was T5 (increased by 27.6%), followed by T4 (increased by 18.0%). During the whole growth period of maize, compared with CK, the content of soil NH + 4 was generally higher than CK, and only T3 was significantly different at the tasseling and maturity stages ( Figure 1B). were extracted with 2 mol·L −1 KCl and determined by flow analyzer; Pa was extracted with 0.5 mol·L −1 NaHCO3 (pH 8.5) and determined by molybdenum antimony anti colorimetry. The determination methods referred to Bao [29].

Plant Harvest and Sample Treatment
After the wheat was mature, the yield components of wheat were investigated by the "1 m double row method". Ten representative plants were randomly selected from each plot and divided into three parts (grain, stem, and leaf), washed, bagged, dried at 80 °C to constant weight.
When the maize was harvested, 10 representative plants were randomly selected in each plot and divided into stalk and grains, then washed, bagged, dried at 80 °C to constant weight.
All of the collected samples were ground and mixed evenly to obtain analytical samples and determine the plant nitrogen and phosphorus.

Data Processing
The data in the dynamic figure and column chart about nitrogen in soil were from 2020 to 2021. The crop yield and nutrient accumulation were the mean values of two rotations from 2019 to 2021. Office 2017 and Origin (OriginLab, Mass, USA) were used for the data processing and plotting. The least significant difference (LSD) was used for multiple comparisons between the different treatment means.

Soil NH + 4 under Different Combinations of N, P Synergists in Wheat/Maize Rotation
From the variation trend during the wheat season, the NH + 4 content in the 0~20 cm soil layer was the highest at the seedling stage, and decreased to about 50% of the seedling stage at the heading and maturity stage in the next year ( Figure 1A), which was consistent with the trial results in 2019-2021. At the wheat seedling stage, the NH + 4 content treated with T3 (HQ + DMPP + biochar) and T6 (NBPT + DMPP + CMFs) were the highest two, increased by 7.65% and 9.21% compared with CK, respectively. At the wheat heading stage, the NH + 4 levels in all treatments dropped to half of the seedling level, but the soil NH + 4 level under T1, T3, and T6 increased significantly compared with CK, with increases of 35.5%, 45.8%, and 50.1% ( Figure 1A). At the maturity stage of wheat, the content of soil NH + 4 increased slightly compared with the heading stage, while the largest one was T5 (increased by 27.6%), followed by T4 (increased by 18.0%). During the whole growth period of maize, compared with CK, the content of soil NH + 4 was generally higher than CK, and only T3 was significantly different at the tasseling and maturity stages ( Figure 1B).  Two-year observations showed that the interannual variability of NH + 4 in the surface soil was much higher than that of the deeper soil ( Figure 2). At the initial stage of fertilization, the difference of NH + 4 at different depths was the largest, and then the difference of NH + 4 in vertical direction gradually became smaller with the prolongation of the growth period. The deeper the soil, the lower the NH + 4 content. The change trend of each treatment between 20~40 cm and 40~60 cm was similar to that of 0~20 cm, indicating that NH + 4 in the deep soil was mainly derived from the leaching and downward movement of the top soil. N, P synergists could reduce the generation of NH + 4 to a certain extent, which eased the NH + 4 leaching loss. Two-year observations showed that the interannual variability of NH + 4 in the surface soil was much higher than that of the deeper soil (Figure 2). At the initial stage of fertilization, the difference of NH + 4 at different depths was the largest, and then the difference of NH + 4 in vertical direction gradually became smaller with the prolongation of the growth period. The deeper the soil, the lower the NH + 4 content. The change trend of each treatment between 20~40 cm and 40~60 cm was similar to that of 0~20 cm, indicating that NH + 4 in the deep soil was mainly derived from the leaching and downward movement of the top soil. N, P synergists could reduce the generation of NH + 4 to a certain extent, which eased the NH + 4 leaching loss. under the treatments with added N, P synergists was generally higher than that of CK. However, there was no statistical significance. At the wheat heading stage, only in the T2 treatment (HQ + DCD + CMFs) was the NO -3 significantly higher than CK; at the wheat maturity stage, only in the T5 treatment (NBPT + DMPP + biochar) did NO -3 show a significant difference with CK. With the extension in the growth period, the NO -3 in 0~20 cm layer showed a continual decreasing trend ( Figure 3A).

Soil NO −
3 under Different Combinations of N, P Synergists in Wheat/Maize Rotation During the overwintering period of the wheat season, the decrease trend of NO − 3 in the 0~20 cm soil layer was similar to NH + 4 , with an overall decrease of 40-50% at the tassel and harvest stages compared to the seedling stage. At the wheat seedling stage, the NO − 3 under the treatments with added N, P synergists was generally higher than that of CK. However, there was no statistical significance. At the wheat heading stage, only in the T2 treatment (HQ + DCD + CMFs) was the NO − 3 significantly higher than CK; at the wheat maturity stage, only in the T5 treatment (NBPT + DMPP + biochar) did NO − 3 show a significant difference with CK. With the extension in the growth period, the NO − 3 in 0~20 cm layer showed a continual decreasing trend ( Figure 3A).
In each growth period of maize, the soil NO − 3 treated by N, P synergists was generally lower than that of CK. The urease/nitrification inhibitor that coupled the phosphorus activator with basal fertilizer could effectively reduce the conversion from NH + 4 to NO − 3 , and consequently reduce NO − 3 accumulation ( Figure 3B). The content of NO − 3 in the soil decreased steadily during the whole rotation growth period of wheat and maize (Figure 4). At the initial stage after fertilization, the difference between the upper and lower soil layers was the largest, and then the difference decreased significantly with the wheat harvest and maize harvest. With the increase in soil depth, the NO − 3 content gradually decreased. From 56 d after fertilization to the heading stage of wheat, the time span was more than five months, and NO − 3 showed a serious leaching phenomenon in the 0~60 cm depth soil. After maize harvest, NO − 3 under different N, P synergist combinations was significantly lower than CK, and the trend was similar  In each growth period of maize, the soil NO -3 treated by N, P synergists was generally lower than that of CK. The urease/nitrification inhibitor that coupled the phosphorus activator with basal fertilizer could effectively reduce the conversion from NH + 4 to NO -3 , and consequently reduce NO -3 accumulation ( Figure 3B). The content of NO -3 in the soil decreased steadily during the whole rotation growth period of wheat and maize (Figure 4). At the initial stage after fertilization, the difference between the upper and lower soil layers was the largest, and then the difference decreased significantly with the wheat harvest and maize harvest. With the increase in soil depth, the NO -3 content gradually decreased. From 56 d after fertilization to the heading stage of wheat, the time span was more than five months, and NO -3 showed a serious leaching phenomenon in the 0~60 cm depth soil. After maize harvest, NO -3 under different N, P synergist combinations was significantly lower than CK, and the trend was similar among the 0~20 cm, 20~40 cm, and 40~60 cm depth soils. These results identified that the nitrifying inhibitors combined with biochar or CMFs could play a sustainable role, significantly inhibiting NO -3 from generating, but the retarding-holding effect on soil NO -3 was not obvious.   In each growth period of maize, the soil NO -3 treated by N, P synergists was generally lower than that of CK. The urease/nitrification inhibitor that coupled the phosphorus activator with basal fertilizer could effectively reduce the conversion from NH + 4 to NO -3 , and consequently reduce NO -3 accumulation ( Figure 3B). The content of NO -3 in the soil decreased steadily during the whole rotation growth period of wheat and maize (Figure 4). At the initial stage after fertilization, the difference between the upper and lower soil layers was the largest, and then the difference decreased significantly with the wheat harvest and maize harvest. With the increase in soil depth, the NO -3 content gradually decreased. From 56 d after fertilization to the heading stage of wheat, the time span was more than five months, and NO -3 showed a serious leaching phenomenon in the 0~60 cm depth soil. After maize harvest, NO -3 under different N, P synergist combinations was significantly lower than CK, and the trend was similar among the 0~20 cm, 20~40 cm, and 40~60 cm depth soils. These results identified that the nitrifying inhibitors combined with biochar or CMFs could play a sustainable role, significantly inhibiting NO -3 from generating, but the retarding-holding effect on soil NO -3 was not obvious.

Alkali-Hydrolyzable Nitrogen under Different Combinations of N, P Synergists in Wheat-Maize Rotation System
At the 0~20 cm depth soil layer, the Nah content in each synergist treatment during the whole growth period was generally higher than that of CK ( Figure 5). At the wheat seedling stage, the T3 treatment (HQ + DMPP + biochar) increased the Nah content by 18.5% compared to CK; at the wheat heading stage, the T3, T5 (NBPT + DMPP + biochar), and T6 (NBPT + DMPP + CMFs) treatments increased by 12.5%, 13.9%, 14.2% more than CK, respectively; at the mature period of wheat, the Nah content in the each synergist treatment was still generally higher than that of CK. Throughout the whole growth period At the 0~20 cm depth soil layer, the Nah content in each synergist treatment during the whole growth period was generally higher than that of CK ( Figure 5). At the wheat seedling stage, the T3 treatment (HQ + DMPP + biochar) increased the Nah content by 18.5% compared to CK; at the wheat heading stage, the T3, T5 (NBPT + DMPP + biochar), and T6 (NBPT + DMPP + CMFs) treatments increased by 12.5%, 13.9%, 14.2% more than CK, respectively; at the mature period of wheat, the Nah content in the each synergist treatment was still generally higher than that of CK. Throughout the whole growth period of wheat, the synergist combinations could effectively increase the soil Nah, and T3 showed the most obvious effect. At the jointing stage of maize, the Nah content in T4 was the highest, with an increase of 16.8%; in the tasseling stage and maturity period of maize, the Nah content under synergist treatment was generally higher, but only in T5 was the difference statistically significant; in the mature stage, the Nah in T5 was still the most obvious. Throughout the whole growth period of maize, the Nah in each synergist treatment had a general increasing trend compared with CK, and Nah in T3 remained stable and maintained a higher level all along.
From the overall view in the 0~60 cm soil, the Nah gradually decreased as the soil layer deepened, and the synergist combinations significantly took on a common positive effect ( Figure 6). Compared with the previous results of urease/nitrification inhibitors or phosphorus activator alone, the Nah rose more obviously under the combinations of the N, P synergists. At the initial stage of fertilization, in the vertical direction, the soil Nah of 0~20 cm was significantly higher than that of the 20~40 cm and 40~60 cm, and the Nah in each synergist treatment was significantly higher than that of CK. After the maize was harvested, the Nah of in the 20~40 cm and 40~60 cm depth soils showed a decreasing trend. Whether it was in the wheat season or in the maize season, the soil alkaline nitrogen content of each synergist treatment was generally higher than CK, indicating that the combinations of N, P synergists could play a sustainable and multiple role in stabilizing the available nitrogen fertility. From the temporal and spatial variation trend of Nah in the early fertilization stage, the higher the content of Nah in the surface soil, the more serious the downward leaching in the vertical direction after the stage of wheat and maize harvest. At the jointing stage of maize, the Nah content in T4 was the highest, with an increase of 16.8%; in the tasseling stage and maturity period of maize, the Nah content under synergist treatment was generally higher, but only in T5 was the difference statistically significant; in the mature stage, the Nah in T5 was still the most obvious. Throughout the whole growth period of maize, the Nah in each synergist treatment had a general increasing trend compared with CK, and Nah in T3 remained stable and maintained a higher level all along.
From the overall view in the 0~60 cm soil, the Nah gradually decreased as the soil layer deepened, and the synergist combinations significantly took on a common positive effect ( Figure 6). Compared with the previous results of urease/nitrification inhibitors or phosphorus activator alone, the Nah rose more obviously under the combinations of the N, P synergists. At the initial stage of fertilization, in the vertical direction, the soil Nah of 0~20 cm was significantly higher than that of the 20~40 cm and 40~60 cm, and the Nah in each synergist treatment was significantly higher than that of CK. After the maize was harvested, the Nah of in the 20~40 cm and 40~60 cm depth soils showed a decreasing trend. Whether it was in the wheat season or in the maize season, the soil alkaline nitrogen content of each synergist treatment was generally higher than CK, indicating that the combinations of N, P synergists could play a sustainable and multiple role in stabilizing the available nitrogen fertility. From the temporal and spatial variation trend of Nah in the early fertilization stage, the higher the content of Nah in the surface soil, the more serious the downward leaching in the vertical direction after the stage of wheat and maize harvest.

Available Phosphorus under Different Combinations of N, P Synergists in Wheat/Maize Rotation System
Compared with CK, the available phosphorus (Pa) at the 0~20 cm soil layer under each synergist treatment was significantly increased. Especially in the early stage of fertilization, the activating effect was the most obvious and the difference between treatments was the largest (Figure 7). At the seedling stage of wheat, the Pa in T2 and T6 was significantly different from that of CK, increasing by 41.3% and 34.5%, respectively; at the heading stage, except T4 (HQ + DMPP + CMFs), P a content in the other treatments all were significantly higher than that of CK, T5 and T6, increase of 15.5% and 14.8% respectively; At the wheat maturity, the Pa in T1 (HQ + DCD + biochar) and T2 was the highest, respectively, increasing by 24.8% and 22.5%, significantly higher than that of CK.

Available Phosphorus under Different Combinations of N, P Synergists in Wheat/Maize Rotation System
Compared with CK, the available phosphorus (Pa) at the 0~20 cm soil layer under each synergist treatment was significantly increased. Especially in the early stage of fertilization, the activating effect was the most obvious and the difference between treatments was the largest (Figure 7). At the seedling stage of wheat, the Pa in T2 and T6 was significantly different from that of CK, increasing by 41.3% and 34.5%, respectively; at the heading stage, except T4 (HQ + DMPP + CMFs), Pa content in the other treatments all were significantly higher than that of CK, T5 and T6, increase of 15.5% and 14.8% respectively; At the wheat maturity, the Pa in T1 (HQ + DCD + biochar) and T2 was the highest, respectively, increasing by 24.8% and 22.5%, significantly higher than that of CK. At the jointing stage of maize, compared with CK, the Pa content at the 0~20 cm soil in T1, T3, T5, and T6 was significantly higher than that of CK, and the highest one was T6, increasing by 23.0%; at the heading stage of maize, the Pa content in T3 was the highest,

Available Phosphorus under Different Combinations of N, P Synergists in Wheat/Maize Rotation System
Compared with CK, the available phosphorus (Pa) at the 0~20 cm soil layer under each synergist treatment was significantly increased. Especially in the early stage of fertilization, the activating effect was the most obvious and the difference between treatments was the largest (Figure 7). At the seedling stage of wheat, the Pa in T2 and T6 was significantly different from that of CK, increasing by 41.3% and 34.5%, respectively; at the heading stage, except T4 (HQ + DMPP + CMFs), Pa content in the other treatments all were significantly higher than that of CK, T5 and T6, increase of 15.5% and 14.8% respectively; At the wheat maturity, the Pa in T1 (HQ + DCD + biochar) and T2 was the highest, respectively, increasing by 24.8% and 22.5%, significantly higher than that of CK. At the jointing stage of maize, compared with CK, the Pa content at the 0~20 cm soil in T1, T3, T5, and T6 was significantly higher than that of CK, and the highest one was T6, increasing by 23.0%; at the heading stage of maize, the Pa content in T3 was the highest,  CK  T1  T2  T3  T4  T5  T6   20   40   60   Soil depth (cm)   CK  T1  T2  T3  T4  T5  T6   20   40   60   Soil depth (cm)   CK  T1  T2  T3  T4  T5  At the jointing stage of maize, compared with CK, the Pa content at the 0~20 cm soil in T1, T3, T5, and T6 was significantly higher than that of CK, and the highest one was T6, increasing by 23.0%; at the heading stage of maize, the Pa content in T3 was the highest, increasing by 28.6%; and at the mature stage of maize, the Pa content in T2 was the highest, increasing by 21.5%.
The Pa content showed a sharply decreasing trend as the depth of the soil layer increased in the vertical direction of the 0~60 cm soil (Figure 8). After fertilization, the difference of Pa among the treatments in the 0~20 cm soil layer gradually decreased with time. The difference between the 20~40 cm and 40~60 cm was the smallest on day 224 after fertilization. From d 224 to d 344, the Pa content in the 0~20 cm surface soil showed an upward trend, while the Pa content in the 20~40 cm soil layer was relatively steady, but the Pa content of the 40~60 cm soil layer showed a downward trend. These results indicate that the combinations of N, P synergists could improve the soil Pa to varying degrees and block the migration of Pa to deeper layers. ference of Pa among the treatments in the 0~20 cm soil layer gradually decreased with time. The difference between the 20~40 cm and 40~60 cm was the smallest on day 224 after fertilization. From d 224 to d 344, the Pa content in the 0~20 cm surface soil showed an upward trend, while the Pa content in the 20~40 cm soil layer was relatively steady, but the Pa content of the 40~60 cm soil layer showed a downward trend. These results indicate that the combinations of N, P synergists could improve the soil Pa to varying degrees and block the migration of Pa to deeper layers.

Effects of Different Combinations of N, P Synergists on the Production of Wheat-Maize
The N, P synergists could effectively improve the bioaccumulation of wheat and maize (Table 1). Compared with CK, the yield inT3 and T6 showed the most obvious effect: the wheat grain was increased by 20.2% and 21.0%, and the maize grain increased by 21.4% and 14.6%, respectively. In the T3 treatment, the total grain yield and the total straw were steadily the highest (24.2% higher than CK), followed by T1 and T2, which were 10.5% and 13.7% higher, respectively, than CK.  CK  T1  T2  T3  T4  T5  T6 344 Days Figure 8. The temporal and spatial variation of available phosphorus under different combinations of N, P synergists during the whole rotation cycle of wheat/maize.

Effects of Different Combinations of N, P Synergists on the Production of Wheat-Maize
The N, P synergists could effectively improve the bioaccumulation of wheat and maize (Table 1). Compared with CK, the yield inT3 and T6 showed the most obvious effect: the wheat grain was increased by 20.2% and 21.0%, and the maize grain increased by 21.4% and 14.6%, respectively. In the T3 treatment, the total grain yield and the total straw were steadily the highest (24.2% higher than CK), followed by T1 and T2, which were 10.5% and 13.7% higher, respectively, than CK. The same alphabet on the right side of the same list show no significance (p > 0.05), in contrast, having significance (p < 0.05), same below.

Nitrogen and Phosphorus Accumulation of Wheat/Maize under Different Combinations of N, P Synergists
The N, P synergists could improve the absorption and accumulation of N and P in wheat, with the increase in N accumulation by 6.11~23.7%, and P accumulation by 9.65~35.1% (Table 2) compared to CK. In T3 and T6, the transporting ratio of N and P to the grain showed the highest: N increased by 33.3% and 35.2% and P increased by 46.8% and 40.4%, both extremely significant. The N accumulation trend in maize was similar to that in wheat; the highest T3 and T6 increased by 27.2% and 17.2%, respectively. The highest phosphorus accumulation was still T3, which increased by 25.5%. Overall, the accumulation of N and P in maize grains under different N, P synergists was generally higher than the control, indicating the synergist combination promoted nutrients absorbed transport into the grains, and thus improved the utilization efficiency of N and P.
In terms of the wheat/maize rotation system, the most N accumulation and the most P accumulation were both in T3 (HQ + DMPP + biochar), 25.7% and 30.8% higher than CK, respectively. The increased accumulation of N and P in wheat was mainly reflected in grain, where nitrogen was particularly prominent.

Distribution of Available Nitrogen and Phosphorus in the Vertical Soil Profile under Different Synergist Combinations
The available nitrogen and phosphorus in the 0~60 cm soil layer showed a certain degree of leaching downward, but the N, P synergists could partly alleviate the trend. The leaching proportion of nitrogen and phosphorus was generally reduced (Figure 9). Under different combinations of N, P synergists, the total amount of NO − 3 in the 0~60 cm soil decreased significantly, with a decrease range of 16.8~32.1%, while the total amount of NH + 4 , Nah, and Pa partially increased. The increases in NH + 4 and N ah in T3 were the highest (23.6% and 15.8% higher than CK), and the increase I the total P in T2 was the highest, followed by T3, with an increase of 46.7%, 30.9% compared to CK, respectively. Judging by the distribution of the main rapid-available nutrients in the vertical profile of 0~60 cm, the incrementally available nutrients were mainly concentrated in the 0~20 cm and 20~40 cm layers, and the distribution ratio in the 40~60 cm soil showed a general downward tendency. The three-dimensional distribution pattern indicated that N, P synergists could effectively hold nutrients around the roots and simultaneously retard the available nutrient leaching.

Retention of N, P Synergists Combined with Basic Fertilizer on Available Nutrients
This experimental region was normal irrigation farmland in the Huang-Huai-Hai Plain. The overall soil fertility was middle-upper level. Corn straw was returned to the field, and wheat no-tillage seed fertilizer with sowing. All maize straw was smashed and returned to soil, mixed with basal fertilizers, then deep ploughed. Wheat seed were sown with basal fertilizer at the same time. Obviously, in order to reduce the risk of yield reduction caused by nutrient deficiency, whether traditional private households or new